The present invention relates to capacitive sensing of topological variations in the structure of an object. The invention particularly relates to a sensing device sensing topological variations in the structure of a finger or a fingertip, and to a so-called fingerprint recognition system.
Various sensing devices, particularly devices intended for sensing topological variations in the structure of a finger or a fingertip are known, which are based on capacitive sensing. One capacitive sensing device is known from U.S. Pat. No. 5,325,442. This device comprises an array of sensor elements which are connected to a drive circuit. Each sensor element comprises a sense electrode, and the array of sense electrodes is covered by a dielectric material defining a sensing surface. Each sensor element includes a switching device which is connected to its sense electrode which is actively addressable by the drive circuit controlling the operation of the switching device. In this manner, a predetermined potential can be applied to the sense electrode. The sensing device still further includes a sensing circuit for sensing a capacitance produced by individual finger surface portions in combination with respective sense electrodes when a finger is placed over the sensing surface. Active addressing of the sensing elements is enabled through the switching devices in each sensing element, which is intended to simplify the driving of the array. It is an object of the invention described in U.S. Pat. No. 5,325,442 to reduce the number of connections and the number of addressing conductors, as compared to other devices. This device is based on reflective sensing.
It could be said that a drawback of the device of U.S. Pat. No. 5,325,442 is that it offers a relatively low sensitivity, in that only one sensor element at a time can be used for sensing. An additional drawback of this device is that since one and the same element is used for transmitting and sensing, there is a risk that the element, when transmitting, will influence the element when sensing, in an undesired manner, by means of, for example, parasitic capacitances. Another problem with this device might be that sweat remaining on the sensor surface could influence the sensing.
SE-C448 408 also shows a capacitive sensing device which comprises a number of sensor elements arranged in a two-dimensional matrix. The object of the invention, as disclosed in said document, is to provide a device enabling a faster sensing and a device reducing the risk of the sensed pattern being influenced by the object, i.e. the finger, moving during the sensing operation. This device is also based on reflective sensing principles.
Still another sensing device is disclosed in EP-A-0 791 899. This device is based on capacitive sensing using direct sensing principles.
However, all sensing devices which are known so far suffer from drawbacks as far as the sensitivity is concerned, i.e. a higher sensitivity is sometimes needed, and furthermore, the sensor area, which particularly contains silicon, which is expensive, is not used as efficiently as would be desired. Furthermore, known sensing devices suffer from the drawback of being inflexible in use.
What is needed is therefore a sensing device which can be given the required sensitivity, which is efficient, and which is cost-efficient from a fabrication point of view. A sensing device is also needed which can be made small and still be highly efficient. Particularly, a sensing device for sensing topological variations in the structure of a finger or a fingertip is needed which meets the above-mentioned requirements, as well as a fingertip pattern or a fingerprint recognition device which here means a device comparing stored information about the variations in the topological structure of a finger with the corresponding information of a finger of a user wanting access to something.
In addition, what is needed is a sensing device with increased sensitivity as compared to prior art, with the possibility of adaptively controlling the sensitivity of the device from case to case. Also, in the case of a device with transmitting elements and sensing elements, what is needed is a sensing device in which the sensing elements can be shielded or insulated from the transmitting elements.
A method is also needed through which the topological variations in the structure of an object can be detected in an appropriate way, with the desired sensitivity and also in an easy and cost-efficient manner.
Therefore a capacitive sensing device is provided which is intended for sensing topological variations in the structure of an object. One example of such an object is a finger used in finger(tip) pattern recognition systems etc. to establish whether a user should be given access to something or not. Particular implementations will be described below. The sensing device comprises a sensing surface for receiving an object adjacent thereto and a number of sensor elements which are arranged in two dimensions in a matrix or in an array. The sensing device comprises controlling means for providing control signals to the sensor elements. At least a number of the sensor elements can be controlled or brought to have or to assume one of at least two different functionalities, and the actual functionality of a sensor element is controlled or determined by said control signals. Any of said at least two functionalities can be assumed by any one of the controllable sensor elements, and any other of said at least two functionalities can be assumed by any one of the other controllable sensor elements.
In a particular embodiment, any of said at least two functionalities can be assumed by any one of the controllable sensor elements, and any other of said at least two functionalities can simultaneously be assumed by any one of the other controllable sensor elements.
In addition, in one embodiment, any of said at least two functionalities can be assumed by any number of said controllable sensor elements, and any other of said at least two functionalities can be assumed by any number of the remaining controllable sensor elements.
The present invention also enables any of said at least two functionalities to be able to be assumed by any number of said controllable sensor elements, and any other of said at least two functionalities can simultaneously be assumed by any number of the remaining controllable sensor elements.
The functionalities available for the at least a number of controllable sensor elements which can be brought to take one of at least two different functionalities comprise a driving functionality in which a sensor element acts as a transmitting element and a sensing functionality in which a sensor element acts as a receiving element. This means that the sensor elements, or at least a number of the sensor elements, are bi-directional, which means that they can be selected to act either as a transmitter or as a receiver.
Due to the flexibility of the sensing device of the present invention, the sensitivity of the device can be varied adaptively from case to case, since the number of transmitting elements as well as the number of sensing elements can be varied from case to case. Additionally, since any element can be used as transmitting element, with any of the other elements being used as the corresponding sensing element, the distance between transmitter and receiver (sensor) can be varied at will, thus raising the level of isolation between transmitter and receiver. In addition, since the distance between the transmitting element(s) and the receiving element(s) can be chosen freely, the problem of remaining sweat on the sensor surface can be obviated.
In a further embodiment, at least a number of the controllable sensor elements can be controlled to assume a third functionality. Preferably, all sensor elements are operable in all said three functionalities and can thus be controlled to have any of at least three different functionalities.
Suitably, said third functionality is an insulating functionality in which a sensor element is inactive, or not selected. Thus, all sensor elements can be controllable, and each sensor element can be controlled to act as a receiving element, a transmitting element, or to be in an insulating, inactive mode, in which mode the element is not selected. This makes it possible to even further raise the sensitivity of the sensing device.
In an alternative embodiment, for all sensor elements the functionalities which are available comprise a sensing functionality as referred to above and the inactive, insulating functionality in which the sensor element is inactive, which also can be expressed as the sensing element not being selected at all.
In one embodiment, at least a number of the sensor elements are controllable via the control signals, and can be brought to act either as a receiving element, a transmitting element or they can be inactive, particularly not selected. In a particularly advantageous embodiment, all sensor elements can be controlled so as to have any of the above-mentioned functionalities, i.e. receiver, transmitter or inactive. Advantageously, however, all sensor elements are operable in at least two different modes and they can be selected to have one of at least two different functionalities.
In a preferred embodiment, the sensor elements are arranged in a two-dimensional matrix comprising X rows and Y columns. In a particular embodiment some of the sensor elements are selected to act as transmitting elements whereas some of the other sensor elements are selected to act as receiving elements. Advantageously, the number of transmitting elements exceed the number of receiving elements. According to this embodiment, the transmitting elements transmit pulsating voltage signals which are provided to the object, for example the finger, the variations in topological pattern of which is to be sensed, and the receiving elements measure the capacitively transferred signal which is reflected back to the sensing device. In other words, the device acts as a reflective sensor.
In an alternative embodiment, conducting means are provided for establishing an ohmic contact between the object and the sensing device. In this embodiment, which relates to a sensor using direct sensing principles, the conducting means acts as a transmitting electrode and a number of sensor elements are selected to act as receiving elements. The remaining sensor elements are selected to be inactive or in other words not selected at all and in this embodiment no elements are selected to act as transmitting elements. Particularly the conducting means is arranged externally and connected to the sensing device. Advantageously however, the conducting means is arranged externally to the sensing matrix but within the sensing device itself.
In a preferred embodiment, via the controlling means a first operational mode can be selected in which the sensing device acts as a reflective sensor or alternatively a second operational mode can be selected in which the sensor acts as a direct sensor. In the latter case conducting means, e.g. of metal, are provided in an appropriate way to enable the establishment of an ohmic contact between the sensing device and the finger or whatever be the object the pattern of which is to be sensed.
The sensor matrix, or at least some of the sensor elements, is/are programmable and the number of sensor elements acting as transmitting elements can be controlled as well as the actual positions of the transmitting elements can be controlled. Thus the functionalities of the sensor elements and the operational mode of the sensing device are controllable (programmable) and can be adapted to the actually prevailing circumstances, e.g. as for as the object is concerned, the ambient conditions etc.
In one embodiment, the controlling mean comprise a first and a second controlling arrangement, each providing at least one digital control signal to each of a number of sensor elements wherein said input signals determine the functionality of the sensor elements. In a particular embodiment, control signals are provided groupwise to a number of sensor elements, e.g. a sub-array or similar of sensor elements. In that case, not all sensor elements are separately controllable, but some of them can be controlled groupwise or elements can be arranged in different groups and the entire control procedure is carried out through controlling groups of sensor elements.
In an exemplary embodiment, the first controlling means comprise a dual shift register and the second controlling means comprise a second dual shift register. The length of the first dual shift register corresponds to the number of rows, whereas the length of the second dual shift register corresponds to the number of columns. Each of the shift registers provide digital control signals via a row interface and a column interface respectively to the sensor elements being controlled, and the result of the control signals provided to the respective sensor element (or group of sensor elements) determines the functionality of the respective sensor element (group of sensor elements).
Each dual shift register provides two digital control signals to each sensor element that is to be controlled. One of said signals indicates whether the sensing functionality is to be activated, whereas the other indicates whether the driving functionality is to be activated. A particular functionality of a sensor element is only activated if the control signals from both dual shift registers actually indicate one and the same functionality for the concerned sensor element.
In a preferred embodiment, the receiving functionality has priority over the transmitting functionality so that if the control signals from both shift registers indicate activation of both the sensing and the driving functionality for a sensor element, the sensor element will act as a receiver.
A method of sensing topological variations in the structure of an object which is received adjacent a sensing surface of a capacitive sensing device is also provided. The capacitive sensing device includes a number of sensor elements arranged in at least two dimensions. The method comprises providing control signals by means of which at least one of the sensor elements is controlled to have the functionality of a receiving element and at least one of said sensor elements is controlled to have the functionality of a transmitting element, generating capacitive or conductive signal(s) to the object via said transmitting element(s), detecting the capacitance between a number of different locations on the object and different receiving elements. The method also comprises using the capacitive signals received in the receiving elements for determining the topological pattern in the structure of the object, and generating an (analogue) output signal describing the topological pattern.
In one embodiment of the method, at least one of said sensor elements is activated to have the functionality of a receiving element, while simultaneously the at least one of said sensor elements is activated to have the functionality of a transmitting element.
In one embodiment the method further comprises the step of storing the information about the topological pattern of the structure of, for example, a finger of the owner of the device or the user entitled to get access to/by the device. Still further the method may comprise the steps of using the sensing device to determine the actual topological pattern of an object at another, later, occasion, comparing the stored information with the actual topological pattern, and generating an output signal indicative of the correspondence between the stored information and the actual information. This output signal can of course take a number of different forms. According to one embodiment an output signal is only generated if the correspondence exceeds a given value or vice versa.
Therefore, a data carrier is also provided, which includes a finger(tip) recognition device comprising a sensing device with a number of controllable sensor elements for sensing the variations in the structure of the topological pattern of a finger or fingertip, storing means for storing such information about the finger/fingertips of entitled user(s), comparing means for comparing input finger/fingertip pattern information of a user attempting/wanting access to, or by, said data carrier and means for generating an acceptance signal if the correspondence between stored information and finger/fingertip pattern information of a user wanting access, at least meets some predetermined criterium or at least reaches a predetermined level. Alternatively a rejection signal may be generated which indicates rejection of the accessing user, i.e. that the correspondence between the stored finger(tip) pattern information is insufficient or particularly lower than a predetermined level, otherwise access is automatically provided. The sensor device is capacitive and comprises controlling means for providing control signals to the sensor elements, and the functionality of each of at least a number of the sensor elements can be selected via the control signals. At least two functionalities are available to, or can by means of the control signals be assumed by, at least a number of said sensor elements.
According to the invention, any of said at least two functionalities can be assumed by any one of the controllable sensor elements, and any other of said at least two functionalities can be assumed by any one of the other controllable sensor elements.
Suitably, any of said at least two functionalities can be assumed by any one of the controllable sensor elements, and any other of said two functionalities can simultaneously be assumed by any one of the other controllable sensor elements.
In one embodiment, any of said at least two functionalities can be assumed by any number of said controllable sensor elements, and any other of said two functionalities can be assumed by any number of the remaining controllable sensor elements.
Preferably, any of said at least two functionalities can be assumed by any number of said controllable sensor elements, and any other functionality can simultaneously be assumed by any number of the remaining controllable sensor elements.
In a particular embodiment the data carrier is portable and comprises a personal card or similar. It can also be implemented in a computer, a (mobile) telephone, a car, for providing access to controlled areas or localities, credit cards, PCMCIA cards for portable equipment etc. With the controlling means each (or a number of) sensor element can be brought to act as a receiving (sensing) element or to act as a transmitting (driving) element or to be inactive.
According to one aspect of the invention, a number of elements are selected to act as transmitters whereas a number of sensor elements are selected to act as receivers and the sensing of the topological pattern of a finger(tip) is effected through sensing the signals reflected back from a number of locations at the finger which means that the sensing device acts as a reflective sensor.
In a further embodiment, at least a number of the sensor elements can be controlled to assume a third functionality. Preferably, each sensor element can be controlled via the control means to assume each of said three functionalities.
Suitably, said third functionality is an insulating functionality, in which a sensor element is inactive, or not selected.
Thus, all sensor elements can be controllable, and each sensor element can be controlled to act as a receiving element, a transmitting element, or to be in an insulating, inactive mode, in which mode the element is not selected. This makes it possible to even further raise the sensitivity of the sensing device in the data carrier according to the invention.
Alternatively, all sensor elements are selected to act as receivers except for the ones being inactive, and conducting means are arranged to provide an ohmic contact with the finger, which conducting means then acts as a transmitter and direct sensing is implemented. In a preferred embodiment, the sensing device supports reflective as well as direct sensing principles and the same sensing device can be used in both sensor modes.
As can be seen, the sensor device is highly flexible, and since the sensor elements are bi-directional (in addition thereto they can be inactive) the sensor area, which for example is made of expensive silicon, can be used efficiently. Furthermore, a larger area is available for transmitting elements which permits a higher sensitivity. Sensor elements which are not active, i.e. sensor elements which are not selected or selected to be inactive, can shield or insulate sensor elements acting as receivers (sensing elements).
Through making the sensing device, or the sensor elements of the sensor device, programmable, the sensitivity can be adjusted depending on the particular circumstances or, for example, in dependence of different properties of the fingers or the fingertips such as for example the thickness of the skin, the degree of humidity or the dryness. The reflective sensing principle or the direct sensing principle can be selected e.g. depending on what is best in a particular situation etc.
The flexibility of the device is high and it is also possible to implement partial scanning such as activating one single column or one single row for detecting the presence of a finger, for rereading important information to save acquisition time or to minimize microprocessor storing requirement; alternatively, of course, two rows or two columns or any combination may be used, such as e.g. any part of the sensing area.